The disclosure relates to a vehicle control device for controlling a vehicle driving device including an internal combustion engine as a driving force source for wheels and a transmission apparatus.
In a vehicle including an internal combustion engine and a transmission apparatus, the driver may perform inertial travel of the vehicle by releasing the accelerator before stopping or during traveling on a gentle downhill. When engagement devices of the transmission apparatus are engaged in inertial travel, a resistance force against the travel is generated. For example, continuous travel on a gentle downhill leads to an increase in fuel consumption. Accordingly, in such a case, control may be made to enter a neutral state (state in which power transmission between the internal combustion engine and wheels is released) in which the transmission apparatus does not form transmission shift stages and does not transfer power. When the driver accelerates the vehicle by operating the accelerator, an appropriate transmission shift stage needs to be formed in the transmission apparatus in the neutral state according to the travel speed and torque of the vehicle.
When engagement devices are engaged to form a transmission shift stage in the transmission apparatus, the rotation speed of the rotary member of the engagement devices close to the internal combustion engine desirably matches the rotation speed of the rotary member close to the wheels within a predetermined range. However, when the vehicle travels on a downhill with the transmission apparatus kept in the neutral state and the travel speed of the vehicle increases, the rotation speed of the wheels increases and the rotation speed of the rotary member of the engagement device of the transmission apparatus close to the wheels exceeds the rotation speed of the rotary member close to the internal combustion engine and the difference between these rotation speeds may increase. To form a transmission shift stage in the transmission apparatus by engaging the engagement devices while preventing a negative torque from being transferred to the wheels in this state, the rotation speed of the input shaft of the transmission apparatus needs to be increased by controlling the output torque of the internal combustion engine to increase the rotation speed of the rotary member of the engagement devices close to the internal combustion engine. That is, until the output torque of the internal combustion engine increases, the rotation speed of the input shaft of the transmission apparatus increases, and the rotation speed of the rotary member of the engagement device close to the internal combustion engine becomes higher than the rotation speed of the rotary member close to the wheels, the engagement device cannot be engaged because the engagement of the engagement device transfers a negative torque to the wheels or a positive torque is not transferred to the wheels because one of engagement devices for forming a transmission shift stage is a one-way clutch, thereby causing a response delay in transferring the driving force to the wheels. As a result, the acceleration felt by the driver may degrade.
In recent years, hybrid vehicles provided with an internal combustion engine and a rotary electric machine as driving force sources have come into practical use. Some of such hybrid vehicles drive one of the front and rear wheels using the internal combustion engine and drive the other of the front and rear wheels using the rotary electric machine. In such vehicles, engine travel using the internal combustion engine or EV (Electric Vehicle) travel using the rotary electric machine can be performed by driving only one of the front wheels and the rear wheels or four-wheel drive travel can be performed in hybrid travel that drives both of the front wheels and the rear wheels. JP-A-2013-180611 discloses, as one example of such vehicles, a hybrid vehicle in which front-wheel-drive is performed during engine travel, rear-wheel drive is performed during EV travel, and four-wheel drive is performed during hybrid travel (see FIGS. 1 and. 2, paragraph 0019, and the like).
As a matter of course, such vehicles make a driving system transition, such as a shift from engine travel to hybrid travel or a shift from EV travel to hybrid travel. During EV travel, the transmission apparatus is set in the neutral state. During a shift from EV travel to hybrid travel, an appropriate transmission shift stage needs to be formed in the transmission apparatus according to the travel speed and torque of the vehicle, the same as above. However, when, for example, the torque becomes insufficient during acceleration in EV travel and a shift to hybrid travel is made, the travel speed of the vehicle also increases. Accordingly, also in such hybrid vehicles, the problem the same as above may occur.